Tracking filters are known which have a frequency pass band that tracks or follows the changing frequency of a signal applied to the input of the filter. Such tracking filters include an active filter and feedback means to control the frequency pass band of the filter in accordance with the frequency of the output signal from the filter. Known tracking filters are limited in their ability to track a noisy input signal which changes rapidly in frequency and which changes in frequency over a large range.
A tracking band-pass filter capable of tracking an input signal having a rapidly changing frequency is described in a patent application Ser. No. 573,034 filed on Apr. 30, 1975, for "Filter Which Tracks Changing Frequency of Input Signal." The application, by Eldon M. Sutphin, Jr., matured into U.S. Pat. No. 3,978,416 on Aug. 31, 1976. The tracking filter includes an active voltage-controlled filter having a frequency control terminal. The output sine wave signal from the filter is translated to a pulse wave which is applied to a frequency-to-voltage converter. The time period between successive pulses is measured by digital means to provide a period-representing voltage which is then inverted to provide a frequency-representing voltage. The frequency-representing voltage is obtained very quickly in the period of one cycle of the signal passed by the filter, and is applied to the frequency control terminal of the filter.